Krypton and xenon are rare gases that are used in a variety of industrial, commercial, and medical applications and are typically recovered from air. Air contains approximately 78.08 percent nitrogen, 20.95 percent oxygen and 0.93 percent argon, on a moisture-free basis. The remainder of the air contains carbon dioxide, heavier hydrocarbons and trace amounts of neon, helium, krypton, hydrogen and xenon. Typically, krypton is present in an amount of about 1.14 part per million by volume and xenon is present in an amount of about 0.087 parts per million by volume.
Krypton and xenon are recovered from the air by cryogenic distillation that involves the steps of compressing, cooling the air and then rectifying the air in distillation column having high and low pressure columns operatively associated with one another in a heat transfer relationship so that an oxygen-rich column bottoms collects in the low pressure column that is used to condense a nitrogen-rich vapor overhead produced in the higher pressure column. The resulting liquid nitrogen is used to reflux both the high and the low pressure column. The krypton and xenon will collect in the oxygen produced in the low pressure column due to the fact that both the krypton and xenon have a lower volatility than oxygen. Consequently, a liquid oxygen stream, removed from the low pressure column will initially be distilled in a distillation column to produce a krypton-xenon-rich stream that can be further processed through a series of distillation steps to produce krypton and xenon products. In such further processing, heavier hydrocarbons that will also collect in the oxygen are removed.
The distillation column used in connection with the initial concentrating of the krypton and xenon from the liquid oxygen stream is generally integrated into the air separation plant itself. An example of this is shown in U.S. Pat. No. 6,378,333 in which a liquid oxygen stream is removed from the low pressure column and then introduced into the top of a distillation column used to concentrate xenon in a bottoms liquid formed within such column. The distillation column is reboiled with nitrogen-rich vapor from the high pressure column that is in turn condensed to serve as reflux to the high pressure column. A portion of the bottoms liquid can be removed, sent to a trap to remove hydrocarbons and then reintroduced into the distillation column. In U.S. Pat. No. 6,694,775, a liquid oxygen stream is removed from the low pressure column and pumped to produce a pressurized liquid oxygen stream. Part of the pumped liquid oxygen stream is partially heated in a heat exchanger and vaporized. The resulting high pressure oxygen vapor is rectified in a distillation column that is refluxed with a remaining part of the pumped liquid oxygen stream. The heat exchanger is used to condense a compressed air stream that, after condensation, is fed into the double column unit. Part of the compressed air can be used to reboil the distillation column. In US Patent Appln. No. 2006/0021380 A1, unlike the other two patents, a stream of crude liquid oxygen derived from bottoms liquid produced in the high pressure column is further refined in an auxiliary distillation column that is reboiled by an argon condenser to condense argon for reflux purposes within an argon column. The residual liquid from the auxiliary distillation column is taken as the krypton-xenon-rich stream.
The need for krypton and xenon has increased over time due to increased demands for the use of such gas in lighting and laser applications. Xenon is also used as an anesthetic. Thus, there also exists the need to retrofit cryogenic air separation plants to recover the krypton and xenon for such applications. The difficulty with such a retrofit is that it is difficult to modify an existing plant with apparatus such as set forth above.
As will be discussed, the present invention solves this problem by providing a process for the production of a krypton-xenon-rich stream that can be effectuated in a free standing apparatus that utilizes oxygen flowing from the plant in an oxygen pipeline.